Researchers led by Professor Dr Georg Juckel at the LWL university clinic at the Ruhr-Universitat Bochum (RUB) in Germany studied the effects of permanent stress on the immune system.

The team focused mainly on a certain type of phagocytes, namely microglia. Under normal circumstances, they repair synapses between nerves cells in the brain and stimulate their growth.

Once activated, however, microglia may damage nerve cells and trigger inflammation processes.

The research showed that the more frequently microglia get triggered due to stress, the more they are inclined to remain in the destructive mode - a risk factor for mental diseases such as schizophrenia.

However, not every individual who is under permanent stress will develop a mental disorder. Juckel's team suspects the cause to go back to the embryonic stage.

US researchers demonstrated as far back as the 1950s that children born of mothers who contracted true viral influenza during pregnancy were seven times as likely to suffer schizophrenia later in life.

The researchers from Bochum confirmed this hypothesis in animal models.

"The embryo undergoes some kind of immune response which has far-reaching consequences and presumably shapes the future immune system," said Dr Astrid Friebe from the LWL clinic.